Fuse wire is calibrated to take a current limit. V=IR, so the resistance in the fuse wire allows a certain amount of current to flow. Resistance acts as a choke and as a result causes the wire to heat. Thats why electric fires, light bulbs and kettles etc work. they get hot when a current flows. The wire heats to distruction at its rated current therefore breaks the circuit when that current is exceeded.This protects the device from high current surge on failure and also human life. Hope this helps